In electric vehicles powered by storage batteries and direct current (DC) series wound electric motors, i.e., motors with armature and field windings serially connected, particular characteristics of the motor make for excellent low speed maneuvering operation, as well as for strong low speed torque for lugging and acceleration. However, series connected motors make for less than ideal operation at higher speeds, and when using the motor for regenerative braking. In present day electric vehicles, electronic power regulators are used to control torque and speed developed by the electric traction motors. Typically, the regulator comprises a time-ratio or chopper circuit which varies the power developed by the motors by controlling the percentage of time that the motors are connected directly to the power source. For maximum mobility, the power source is usually a battery. The regulator also includes apparatus responsive to accelerator position for varying the percentage on-time or mark-space ratio of the chopper circuit.
The chopper type controls for these vehicles typically employ resistor field weakening involving shunting the field winding with a resistor at appropriate speed and torque load points. The resulting improvement in high speed operation is, at best, compromised and since a certain amount of energy is burned up in the field shunting resistor, there is also an attendant efficiency loss. Therefore, it is most advantageous to change to a separately excited field control mode for higher speed operation. When effecting such a change, it is important that the field current/armature current relationship be precisely controlled to avoid jerkiness.
An additional advantage of continuous separate field control over the nearest equivalent in series motor control relates to that of regenerative braking wherein the motor acts as a generator to deliver energy to the battery at times when the load overhauls the motor. In a separately excited motor, such an operating mode is a relatively straight forward controllable process. A series generator, although practical, presents problems in regards to precise control.
In order to reduce the rate of wear on mechanical brakes in electric vehicles it is common practice to implement some form of electrical braking. In a series connected motor, electrical braking may be either dynamic or plug braking. In either case, the field and armature connections are reversed and armature current is dissipated in shunt resistors (dynamic brake) or in the armature itself (plug brake). The tractive effort is modulated by the chopper control so as not to overstress the electrical or mechanical components, as well as the operator and load, and to provide a set level of retarding torque. This method, while very effective in controlling braking of the vehicle, recovers none of the kinetic energy of the system and also takes a heavy toll on operating components, particularly the drive motor, from heavy current dissipation.
Regulation to a desired braking torque under these conditions tends to be inefficient since the armature current is so large with respect to field current that armature reaction disturbs the normal field flux control of the armature circuit. Because a low level of field current excitation at the higher motor speeds produces very large magnitudes of armature current, the control of plugging becomes relatively critical.
On the other hand, regenerative braking works well at a high kinetic energy state, when the vehicle velocity is high, but rapidly looses its effectiveness as speed decreases. Even though regenerative braking can be maintained down to near zero speed, effective braking torque diminishes to a point where auxiliary braking is needed in a practical case. This point occurs at around half base speed or when three-fourths of kinetic energy has been expended. From this point more effective electrical braking can be obtained from plugging.
The problem to be solved in this instance is to provide a system in which the motor can be connected to provide braking torque in the regenerative, separately excited mode at higher motor speeds and to switch to a plugging mode when braking torque requirement exceeds that obtainable from the regenerative connection, i.e., as speed drops, the transitions being as transparent to the vehicle operator as practical.
Another characteristic often observed in electrical vehicles is that during normal travel, when the operator moves his foot from the accelerator, the vehicle coasts with little or no retarding force such as that experienced that internal combustion engine driven vehicles. Although it is desirable for an operator to experience the same vehicle "feel" when operating either an electric vehicle or an internal combustion engine driven vehicle, implementation of retarding torque in a coast mode requires more complex circuitry and has not generally been available in electric vehicles.
An increasingly important operating capability in industrial vehicles is to limit travel speed due to operating environment or situations such as, for example, raised loads, hazardous areas, floor conditions, turning angle, etc. Typical speed governing methods presently in use tend toward limiting an accelerator command signal usually by mechanical travel restriction, and/or electrical override. While providing a measure of speed limiting, the gradability and load carrying capability is often affected adversely. Additionally, such methods provide little, if any, speed limiting when overhauling (down grade) operation is encountered.
It is an object of the present invention to provide speed limiting characteristics in an electric vehicle with little or no load handling and gradability deterioration, and with speed control equally effective with light and/or overhauling loads.
It is an object of the present invention to provide a simplified control circuit for implementing a retarding torque function in an electric motor driven vehicle when an accelerator is released, including enhancing retarding torque as a function of mechanial/hydraulic service brake pedal position.
It is another object of this invention to provide an electric vehicle control system implementing braking torque in a regenerative, separate excited mode, which system will switch to a plugging mode when braking torque requirement exceeds that obtainable from the regenerative connection, the transitions being substantially transparent to a vehicle operator.
It is further object of this invention to provide an electric vehicle control system in which an electric motor is configured to operate in a series mode at its most advantageous characteristic during starting and maneuvering and change to a separately excited, field control mode at higher speed, such that the field/armature current relationship is precisely controlled, allowing operation at higher speed torque point than is possible to obtain from the series motor connection.
It is yet another object of this invention to provide the coast characteristics of an internal combustion engine driven vehicle in an electric vehicle.